1. Field of the Invention
The present invention relates to a method and apparatus for transmitting and receiving RF signals in an amplification system, wherein sound detected by a microphone is transmitted as an RF signal to a remote receiver, audio-amplifier and loudspeaker which reproduces the sound. More particularly, the present invention relates to a classroom amplification system which employs technology found in cordless telephone transmitters/receivers to transmit and receive RF signals.
2. Description of the Related Art
As is well known in audiological and teaching communities, many children in school, although classified as having normal hearing, have transient hearing loss due primarily to otitis media, more commonly referred to as middle ear infection. For reasons not entirely clear, this kind of infection has been increasing in prevalence for at least the past 20 years. The National Center for Health Statistics has developed data that indicates that this affliction, with accompanying mild hearing losses, increased by almost 45% in the period 1981 to 1988 and that the most common cause for doctor visits for children aged 15 or less during the period 1975 through 1990 was otitis media, showing an increase of 150% during that study period. For children less than 2 years of age, the increase in the same period was 224%. For 1990 alone, the number of children 15 or under reported as having otitis media was a staggering 19.8 million according to this same source.
It is well documented that this trend has resulted in some portion of so-called xe2x80x9cnormally hearing childrenxe2x80x9d in classrooms actually being temporarily xe2x80x9chearing impairedxe2x80x9d at least to some degree. One study conducted at a Midwestern suburban school, involving 282 children from six kindergarten and six first grade classrooms, none of whom was classified as hearing impaired, showed a failure rate when screened for hearing loss according to recommended standards as follows:
For a screening in the fall season . . . 33% failed
For a screening in the winter season . . . 34% failed
For a screening in the spring season . . . 27% failed
Thus, at any given time, between 27% and 34% of the children had some degree of hearing loss and, by means of questionnaires filled out by the parents, it was established that these transient hearing losses were directly associated with episodes of otitis media. Other studies of regular classrooms have shown similar results. Even more dramatic, studies in xe2x80x9cspecial needsxe2x80x9d classrooms have indicated the averages rise to about 75% of the children showing similar transient hearing losses with similar etiologies as causes.
Hearing losses in children, particularly in the earlier grades, can lead to poor performance, reduced classroom participation and development of poor learning and/or study skills. Thus, transient hearing loss is a growing problem with extremely serious consequences both for the children suffering hearing loss and for society at large.
Children with known, permanent hearing deficits are usually fitted with hearing aids or so-called xe2x80x9cclassroom trainers.xe2x80x9d This approach to alleviating a hearing loss problem is not practical in the context of transient hearing loss because of the large number of children afflicted, and because of the transient/floating characteristics of the problem, where at any given time it is not known which children are affected.
To address this problem, since roughly 1980, a number of researchers have been experimenting with what have come to be called xe2x80x9cclassroom amplification systemsxe2x80x9d. In essence, these systems depend on some kind of battery operated transmitter and microphone worn by the teacher, a receiver/audio-amplifier installed in the classroom, and a number of loudspeakers arrayed about the classroom. These systems amplify the teacher""s voice throughout the room so all children can hear without strain, even if they have mild, untreated hearing loss. The success of this approach has led to a growing number of companies whose primary focus is to design and install such systems nationwide.
In practice, the unsolved problem of signal interference in classroom amplification systems has limited deployment of such systems. Typically, radio frequency (RF) transmission is used to couple the teacher""s voice to the receiver; however, the reserved band for this kind of hearing assistance system (72 MHz to 76 MHz) is subject to interference from a variety of nearby sources such as television stations, CB transmitters, incidental interference from electrical appliances and the like. Other frequency bands, such as 49 MHz, 216 MHz, and 900 MHz, have been used in an attempt to obtain better performance; however, to date, the interference problem largely has not been solved.
At present, the best of the RF receiver/transmitters in the hearing impaired community incorporate at most 10 to 15 channels of analog narrowband FM capability with no special encoding or other means for minimizing the effects of interfering transmitters. Interference is typically avoided in such receivers by manually switching channels in both the transmitter and receivers independently of one another.
As a result of the interference problem, seldom can more than a few classrooms in any school have RF amplification systems before serious system functional degradation occurs due to interference among the adjacent classrooms. Furthermore, performance of classroom amplification systems may be degraded by interference from intermittent or permanent sources of RF interference in the vicinity of the school, depending on the school""s location. Thus, despite the development of classroom amplification system technology over the course of many years, there remains a long felt need for a classroom amplification system capable of eliminating or mitigating the effects of interference with minimal user involvement.
It is an object of the present invention to provide an improved system and method for solving interference problems in classroom amplification systems and other RF-type amplifications systems.
It is another object of this invention to provide a simple and inexpensive approach, using RF transmitters and receivers from another industry not associated with the hearing impaired, to improve the performance of classroom amplification systems and other RF-type amplifications systems.
It is a further object of the present invention to automate RF channel selection in a classroom amplification system or other RF-type amplification system, such that minimal user involvement is required to identify and switch to a low-interference channel.
It is yet another object of the present invention to adapt the technology employed in cordless telephone systems to classroom amplification systems and other RF-type amplification systems.
It is yet a further object of the present invention to modify the circuitry of a cordless telephone system to broaden the bandwidth of the system so that the circuitry is suitable for use in an amplification system.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
According to the present invention, an RF-type amplification system, such as a classroom amplification system employs interference reduction technology adapted from a cordless telephone system. In particular, the amplifier system includes a portable remote unit which incorporates features of a portable handset of a cordless telephone and a base unit which incorporates features of a base unit of a cordless telephone. Cordless telephones rely on RF transmission between the portable handset and the stationary base unit that is connected by wire to the telephone system and which serves as a battery charger for the handset when not in use. The transmission techniques employed in cordless telephones incorporate a number of features that reduce or eliminate interference. The present inventors have recognized that the technology employed in cordless telephones can be adapted for use in the field of RF-type amplification systems, such as classroom amplification systems.
More specifically, the remote unit of the amplification system of the present invention includes a microphone for detecting a voice, such as that of an instructor, and for generating corresponding electronic voice signals. A transmitter of the remote unit generates RF signals containing the voice signals (which, optionally, can be scrambled by the transmitter) as well as control signals provided by the central processing unit (CPU) of the remote unit. The control signals include a frequency identity code which indicates the remote unit""s transmit frequency, a handshake code which is earlier downloaded from the base unit to the remote unit and which allows the base unit to uniquely identify the remote unit as the source of the transmitted RF signals, and a descrambling code which allows the base unit to descramble the voice signals transmitted within the RF signal. The remote unit further includes an antenna for transmitting the RF signals and for receiving RF signals from the base unit. Control signals received by the remote unit from the base unit are identified by a code detector and sent to the CPU for analysis.
The remote unit can be formed by modifying a conventional remote unit (handset) of a cordless telephone by: removing the packaging (i.e., the handset housing) and the alphanumeric keypad of the handset; removing the earpiece and the built-in microphone; adding an external microphone; and repackaging the function keys, circuitry (e.g., transmitter, receiver, CPU, etc.) and antenna with a housing suitable for attachment to an instructor""s belt or the like. The resulting remote unit includes function keys for activating and deactivating the remote unit (xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d keys), a xe2x80x9cMUTExe2x80x9d key for muting the voice signal output from the base unit, and a xe2x80x9cChannel Scanxe2x80x9d key that allows the user to initiate an automatic channel scanning mode to identify a transmission channel that is substantially free of interference. The remote unit further includes a rechargeable battery which is charged by a battery charger of the base unit.
The base unit includes an antenna which receives the RF signals transmitted by the remote unit and a receiver which detects the RF signals and separates the RF signals into the constituent voice signals and control signals. The control signals are identified by a code detector of the base unit and evaluated by the CPU of the base unit. If the frequency identity code, handshake code and descrambling code transmitted by the remote unit and received by the base unit have the values expected by the base unit, the voice signals are processed through a speech network and provided at a communications interface of the base unit to an audio power amplifier which amplifies the voice signals and provides the amplifies voice signals to room speakers which audibly project the voice signals as sound.
The communications interface of a cordless telephone is designed to receive an xe2x80x9coff-hookxe2x80x9d current from a telephone line, which xe2x80x9coff-hookxe2x80x9d signal activates the system when the handset is removed from its cradle on the base unit. In the amplification system of the present invention, this xe2x80x9coff-hookxe2x80x9d signal is provided by a DC power source which supplies a constant DC voltage to the communications interface whenever the system is in use. The base unit further includes a transmitter for transmitting control signals, but not voice signals, to the remote unit. Thus, there is one-way transmission of voice signals (from the remote unit to the base unit) and two-way transmission of control signals between said remote unit and said base unit.
When the system user (e.g., an instructor) detects an unacceptable amount of interference on the transmission channel, the user can effect automatic selection of a low-interference channel by selecting the xe2x80x9cChannel Scanxe2x80x9d key. Selection of the xe2x80x9cChannel Scanxe2x80x9d key causes the remote unit to send a control signal to the base unit. In response to the control signal, the base unit scans different frequency channels by adjusting the frequency of a local oscillator in the receiver until a channel substantially free of interference is identified. The base unit then transmits this information to the remote unit which begins transmitting at the indicated frequency to avoid interference.
The control signals and the channel scanning features advantageously minimize interference in the amplification system, even where numerous sources of interference are present in the vicinity of the system.
Optionally, the communication circuitry of a cordless telephone can be further modified for use in an amplification system to broaden the bandwidth of the voice signals being transmitted to the base station. According to one approach, the bandpass filter in the remote unit, which limits the bandwidth of the voice signal, can be bypassed. According to another approach, a compressor circuit can be placed upstream of the bandpass filter, and an expander circuit can be place downstream of the receiver in the base unit to obtain a wider bandwidth while still employing the bandpass filter of the original cordless telephone system.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components.